Interesting. I need to look at the poop sheet for Xtol then. Will I find the dilution ratios there? How does it affect developing time, or am I essentially working with fresh developer since it’s been replenished? All these decades of film developing and I have never done this. This must mean I have probably poured a used car’s value worth of perfectly good developer down the drain. Is replenished developer similar to working with different Rodinal dilutions? There’s still so much me to learn. More than a few lifetimes worth when it comes to the breadth and depth of areas there are to explore in photography.
Xtol used with replenishment is
undiluted: stock solution, mixed to envelope directions, is used both for tank solution and for replenisher. Recommendation is 70 ml replenishment per 8x10 equivalent (= 1x120 or 1x135-36), and you can adjust this figure if you find your activity level drifting toward higher or lower contrast; add the measured replenisher to the tank solution bottle, then discard any solution from the development tank beyond what's needed to refill the bottle. Some claim to need to adjust development time as the tank solution seasons (takes about six rolls with normal replenishment to reach a steady state), but I've been using the stock solution times for films I've processed. YMMV, I suspect it depends on how much you lower your personal EI below box speed (I don't), possibly also how tightly you monitor your negatives (i'm on the loose side there).
In general, I only put about 65 ml of developer down the drain for each roll (the missing 5ml gets carried over into the stop bath, and a little of that carries over into the fixer, so the fixer tends to expand slowly). I started this in part because I'm cheap, and in part because I have a sheet film tank that wants 55 ounces (~1.6L) to cover 4x5 -- but with replenishment, four 4x5 sheets still only consume 70ml of developer, as replenisher. If I were using Xtol at the otherwise recommended 1+1, I'd be dumping a minimum (for 35mm or double-loaded 120) of about 125-150 ml of stock solution per roll, and potentially as much as 800 ml to do one or more 4x5 in my Agitank (which would have me using something else for 4x5 -- homemade Parodinal, most likely).
You can, if you choose, still use diluted Xtol, just dilute some of the replenisher for your one-shot process and discard it when you're done. IMO, there's not generally any reason to want to do this, though.
Presuming XT works enough like Xtol, this should work the same with with that (it is known to do so with EcoPro and Fomadon Excel, and with Mytol if your chemicals are free enough of iron and copper).